(1) Field of the Invention
The present invention relates to a process for the production of ferritic stainless steel sheets or strips in which the production steps are simplified and products comparable or superior to products of conventional processes can be obtained.
(2) Description of the Prior Art
Cold-rolled products of ferritic stainless steels have heretofore been produced by box-annealing a hot rolled steel strip coil at 800.degree. to 850.degree. C. batchwise and repeating cold rolling and recrystallization annealing two times in many cases. Since a hot rolled steel strip has a heterogeneous micro-structure, if this strip is directly subjected to cold rolling, a desired formability cannot be obtained and therefore, batchwise diffusion annealing should be conducted for a long time prior to the cold rolling. However, in order to heat a long coil strip uniformly even to the interior portion of the coil and effect diffusion annealing, the coil should be kept in a furnace for more than 40 hours and thus the entire production time becomes very long, with the result that the manufacturing cost is inevitably increased.
As means for eliminating such disadvantage of long-time batchwise diffusion annealing of ferritic stainless steels, proposals have been made on the so-called continuous annealing process in which a coil is uncoiled and is continuously conveyed through a furnace, or sheets are conveyed one at a time through the furnace. In the following strips and sheets are therefor the equivalent of one another because in both cases a single layer of sheet-gauge steel is conveyed continuously through the furnace.
When a hot rolled strip of a ferritic stainless steel is subjected to continuous annealing instead of conventional batchwise annealing, the strip should necessarily be heated at a higher temperature than that adopted in the conventional process, and if this strip is heated at a high temperature, the ferritic steel is transformed into an austenite-ferrite mixed phase structure.
Japanese Patent Publication No. 30008/76 discloses a continuously annealing process in which a hot rolled strip of ferritic stainless steel is heated at a temperature of from 1330.degree. to 1350.degree. C. exceeding the austenite-ferrite mixed phase region for a short time of less than 3 minutes and the heated steel strip is air-cooled or rapidly cooled at an elevated cooling speed. Furthermore, Japanese Patent Publication No. 1878/72 discloses a continuous annealing process in which a hot rolled strip of ferritic stainless steel is heated at a temperature of from 930.degree. to 990.degree. C. where the austenite and ferrite phases co-exist, for a time shorter than 10 minutes and the heated strip is air-cooled or rapidly cooled at an elevated cooling rate. However, in these conventional continuously annealing processes the austenite phase formed at the annealing step is transformed to a martensite phase during the cooling step, and troubles are caused, for example, at the subsequent cold rolling step. They are rupture of a strip at the cold rolling step and intergranular corrosion at the annealing and pickling steps.
In the process disclosed in U.S. Pat. No 2,808,353, occurrence of such troubles is prevented because a hot rolled strip of ferritic stainless steel is heated at a high temperature of 927.degree. to 1149.degree. C. for from 1 to 10 minutes and is then annealed at 760.degree. to 899.degree. C. batchwise.
As the process using an additive element, a process comprising continuously annealing a hot rolled strip of a Ti-added ferritic stainless steel at 950.degree..+-.20.degree. C. for a time shorter than 10 minutes is disclosed in Japanese Patent Application Laid-open Specification No. 84019/73.
As described hereinafter, the present invention is directed to the production of Al-containing ferritic stainless steel sheets or strips. The use of Al as an additive element is discosed in, for example, British Pat. No. 1,162,562 and Japanese Patent Publication No. 44888/76.